1. Introduction
This invention relates to removal of dissolved contaminants from organic solutions. More particularly, this invention relates to removal of dissolved metallic contaminants from organic solutions used in integrated circuit manufacture.
2. Description of the Prior Art
Ultra pure liquids free of particulate, ionic and organic contamination are required for many industrial purposes such as for the manufacture of pharmaceutical and integrated circuits. For example, in the manufacture of high resolution integrated circuits, it is known that many processing liquids come into contact with a bare wafer or a resist coated surface. These include photoresists and treatment chemicals such as organic liquids and aqueous solutions which contain acids, bases, oxidants, and other proprietary ingredients. At least 15 to 50 liquids of various compositions are used to clean wafers, prime surfaces, deposit resists or other polymers, develop, rinse, etch, and strip the resist. It is known that these solutions may be a source of contamination of the integrated circuit wafer that may interfere with its performance. Thus, the reduction or removal of insoluble and soluble contaminants from processing fluids used for the production of integrated circuits before or during use is basic insurance for prevention of damage to the integrated circuit.
Photoresist coating compositions are used extensively in integrated circuit manufacture. Such compositions typically comprise a light-sensitive component and a polymer binder dissolved in a solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986; 5,212,046; 5,216,111; and 5,238,776, each incorporated herein by reference for disclosure of photoresist compositions, processing, and use.
It is known that photoresist coating compositions contain particulate and ionic contaminants. For example, it is known that solid gels or insolubles form in photoresists due to dark reactions. In addition, soluble impurities such as moisture, silicone oils, plasticizers, and metal ions may be present from the manufacture of the resist components and from the packaging containers or dispensing tanks. Trapped bubbles from point-of-use filtration or the shaking of a resist bottle prior to dispensing can lead to defects in resist coatings. In Class 100 clean rooms, airborne particulate counts amount to 3 particles per liter of density of 2. By comparison, liquids contain about 100,000 particles per liter. A particle count of 100,000 per liter seems high, but if translated into a solid of 0.6.mu. in size, this is equivalent to 10 parts per million parts of solution (ppm). A level of 10 ppm amounts to the deposition of 10 mg per liter. Since liquids are heavily contaminated compared to clean room air, effective contaminant removal is essential to the manufacture of such devices.
Ultrafiltration of resist liquids has progressed and manufacturers of resist now supply resist materials filtered through 0.04 .mu.M diameter absolute filters. However, methods for removal of particulates from treatment solutions are not effective for removal of dissolved contaminants from solution such as organic impurities and ionic species. These contaminants can be at least as damaging to an integrated circuit as particulate contamination.
Efforts to remove dissolved cationic and anionic contaminants from treatment solutions used to manufacture integrated circuits are known in the art. For example, one such method is disclosed in International Publication No. WO 93/12152, incorporated herein by reference, which is directed to removing metal ions such as sodium and iron from novolak resins during manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions within the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin.
A method for removal of dissolved ionic metals and non-metals from a photoresist is disclosed in published Japanese Patent Application No. 1228560 published Sep. 12, 1989, incorporated herein by reference. In accordance with the procedures of this patent, a photosensitive resin is passed through a mixed bed of a cation exchange resin and an anion exchange resin to simultaneously remove cation and anionic species from the photoresist solution.
In copending U.S. patent application Ser. No. 08/128,994, filed Sep. 30, 1993, assigned to the same assignee as the subject application and incorporated herein by reference, an improved process is disclosed for removing metallic cations from organic solutions using modified cation exchange resins. In accordance with the process of the invention disclosed therein, the cation exchange resin is modified by replacement of the acid protons on the cation exchange groups with essentially neutral groups such as ammonium or amine groups. Thereafter, an organic solution containing acid labile components may be treated with the modified cation exchange resin to remove metal ions without the formation of undesired by-products caused by attack of acid protons on acid labile groups.
The process described in the above-identified copending application is suitable for the removal of dissolved cations from solutions containing an acid labile solution. However, the process is especially useful for removing mobile metal ions such as sodium and potassium from such solutions but is somewhat less effective in removing heavier metal ions such as iron and chromium.